This invention relates to carburetors and more particularly to carburetors having a diaphragm type fuel pump.
Carburetors are currently used to provide the combustion fuel requirements for a wide range of two-cycle and four-cycle engines including hand held engines, such as engines for chainsaws and weed trimmers, as well as a wide range of marine engine applications. Diaphragm type carburetors are particularly useful for hand held engine applications wherein the engine may be operated in substantially any orientation, including upside-down. These carburetors utilize a fuel-metering diaphragm which is operative to control the delivery of fuel from the carburetor regardless of its orientation. Additionally, some carburetors utilize a diaphragm type fuel pump which is responsive to engine pressure pulses to draw fuel from a fuel supply and to deliver fuel to the fuel metering assembly under pressure. The fuel pump diaphragm defines a fuel chamber on one side which receives liquid fuel and a pressure pulse chamber on its other side in communication with the engine to receive pressure pulses which actuate the fuel pump diaphragm.
In two-stroke engines, the pressure pulse chamber usually communicates with the crankcase and alternately receives negative and positive pressure pulses to actuate the fuel pump diaphragm.
In four-stroke engines, the pressure pulse chamber is communicated with an intake manifold of the engine which provides a predominantly negative or vacuum pressure signal to actuate the fuel pump diaphragm. This pressure signal from the intake manifold contains fuel vapor which may condense to liquid fuel and collect forming a puddle of liquid fuel in the pressure pulse chamber. Undesirably, this puddle of liquid fuel may be dumped directly into the engine intake manifold when the orientation of the carburetor is changed, or may be rapidly drawn into the engine when the engine speed is rapidly reduced from wide open throttle to idle. This results in an excessively rich fuel condition within the engine which severely affects the stability of the engine, especially at idle, and may even cause the engine to stall. Further, the puddle of liquid fuel within the pressure pulse chamber can adversely affect the performance of the fuel pump. These problems are particularly acute in small four-stroke engines which are highly sensitive to a richer than desired fuel and air mixture provided to the engine.
A carburetor for a four-stroke engine has a body which carries a fuel pump diaphragm which defines a fuel pump chamber on one side and a pressure pulse chamber on its other side in communication with the engine to receive pressure pulses which actuate the fuel pump diaphragm to draw fuel into the carburetor and to discharge fuel to a downstream fuel metering assembly under pressure. An air passage communicates at one end with an air supply and at its other end with the pressure pulse chamber to provide an air flow within the pressure pulse chamber which sweeps away, dries out, disperses or aerates any liquid fuel within the pressure pulse chamber to avoid puddling or accumulation of liquid fuel therein.
In one embodiment, a throttle valve carried by the carburetor body for movement between idle and wide open positions controls the flow of fluid through the air passage as a function of the position of the throttle valve. Desirably, the air passage is open when the throttle valve is in its idle position to provide the air flow into the pressure pulse chamber and to prevent liquid fuel from puddling in the pulse chamber so that liquid fuel is not dumped into the engine intake manifold from the pressure pulse chamber. Due to the large magnitude of the vacuum communicated with the pressure pulse chamber when the engine is idling, the flow of air into the pressure pulse chamber from the air passage does not significantly or materially affect the performance of the fuel pump. Conversely, at wide open throttle the flow of air into the pressure pulse chamber may adversely affect the efficiency of the fuel pump which needs to pump significantly more fuel than at idle to satisfy the engine""s fuel demand at wide open throttle. Therefore, in at least some applications, it is desirable to close off the air passage when the throttle valve moves to its wide-open position to avoid adverse affects on the diaphragm fuel pump. At high engine speeds, if liquid fuel collects within the pressure pulse chamber and is discharged therefrom into the engine, the engine is not likely to stall because it is more tolerant of a rich fuel mixture when operating at wide open throttle and high speed conditions.
Objects, features and advantages of this invention include providing a carburetor which is ideally suited for small four-stroke engines, reduces or eliminates puddling of liquid fuel in a pressure pulse chamber of the diaphragm fuel pump at least during idle engine operation, eliminates a puddle of fuel from being dumped into the intake manifold at least during idle engine operation, permits the engine to be initially started and operated with a richer fuel and air mixture desirable for starting and warming up of the engine, increases the tolerance of the carburetor to be operated in substantially any orientation even during idle engine operation, does not significantly effect the performance of the fuel pump, provides more consistent fuel pump performance, improves the idle operation and stability of the engine, eliminates engine stall when the engine is rapidly changed from wide open throttle operation to idle operation, is applicable to substantially any carburetor design, is of relatively simple design, economical manufacture and assembly, rugged, reliable, durable and has a long useful life in service.